This invention relates to a process and apparatus for the removal of heavy metals from wastewater being treated.
It is known that heavy metals are present in various types of wastewaters. Of the various types of wastewaters, industrial wastewaters are known to contain especially high concentrations of heavy metals.
In the prior art it is known to separate heavy metals from wastewaters from the metalworking industry by various precipitation methods, electrolysis, ion exchange, etc., so that the amount of heavy metals in these wastewaters are thereby reduced to small residual concentrations. In addition to the above, heavy metals are also found, for example, in distiller's washes of the brandy production industry, and in wastewaters from the pulp industry. In both of these instances, the wastewaters carry both an inorganic load as well as a high organic load, with the organic load containing heavy metals. As a result, the above-mentioned precipitation and other chemical purification processes are not effective since the organic load prevents heavy metals removal e.g. by buffering, complexation, competitive reactions, emulsifying properties etc. These prior art processes are discussed in the text "Wastewater Engineering" by Metcalf and Eddy; McGraw Hill 1979; pages 753-756 whose disclosure is incorporated herein by reference.
This same problem occurs when industrial wastewaters loaded with heavy metals are fed, in combination with organic-loac-carrying municipal wastewaters, to a wastewater purification/clarification plant to be processed therein, Although the organic load can be reduced, a disturbing result of the conventional methods is that there are no practical methods permitting the elimination of the heavy metals from these wastewaters carrying the high organic load. Hereinafter, the terms "purification", "treatment", or "clarification" as applied to wastewater treatment plants are intended to mean the same thing and refer to conventional municipal installations.
These heavy metals exert a deleterious influence in such wastewaters because the activated sludge, and later on, the digested sludge become enriched with the heavy metals. As a result, the heavy metal concentration in the digested sludge or in the aerobically stabilized sludge exceeds specified maximum values set by various regulations. laws, etc., then the sludge can no longer be passed directly to the agriculture industry for use in, e.g., fertilization of soil, but instead must be disposed of in other ways. Disposal of the heavy metals enriched digested sludge requires special dumps, which result in significantly higher costs than if the digested sludge is merely released directly for agricultural use.